Two major small intestinal cell surface digestive enzymes, lactase and sucrase, will be isolated from human intestine by affinity chromatography and characterized by conventional techniques of protein chemistry. In addition, production of specific antisera in rabbits will allow radioimmunoassay of the enzymes, their precursors, and their degradation products so that it may be possible to define the defect in hereditary deficiency of these two disaccharides. The mechanisms of synthesis and degradation of sucrase will be studied in vivo in rats by specific quantitative immunoprecipitation of pulse labeled glycoprotein with consideration of both the route of administration and of the type (amino acid or hexose) of radioactive precursor, and the effect of alloxan-induced diabetes on turnover. After labeling of rat small intestinal surface proteins in vivo with I125, the fate of the enzyme will be studied by isolation and characterization of immunoprecipitable I125 proteins using the specific anti-sucrase antiserum. Finally, the human intestinal crypt immunosucrase that is devoid of enzyme activity will be isolated and its structure compared with that of the normal villus enzyme in order to elucidate whether the cross-reacting crypt material could be a precursor of the mature villus sucrase.